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Multisim does not include an antenna component. It seems that a basic approach would be to use a voltage source in series with a resistor (antenna resistance). Another idea would be the use of an equivalent circuit consisting of a capacitor, inductor and resistor.
Also, in Multisim take a look of the RF Group in the Master Database. It contains some components that could be useful for your simulation.
what should be the value of the voltage regulator if the antenna resistance is 50ohm
A single sentence added to a 6 year old thread is not sufficient to help you. Please provide significantly more detail. What does an antenna have to do with voltage regulation?